The present invention is related to a steam reforming method for methanol for obtaining hydrogen gas and a system therefor. More specifically, a method of the present invention and system therefore utilizes steam and methanol as raw materials for producing hydrogen gas.
Hydrogen gas has been used in such various processes as reforming process and desulfurizing process in petroleum refinery, and synthetic process and hydrogen annexing process in chemical industries. Recently, hydrogen is becoming to be used in more various industrial areas, for example, as a cover gas of epitaxial furnace or apparatus for fabrication of silicon monocrystal, in fabrication of magnetic iron fillings for audio or video tapes in the field of microelectronics, in fabrication of margarine, shortening and synthetic sweetening in the field of food industry, etc. Thus, hydrogen gas is becoming to be demanded in more and more wide varied industrial and technical fields. Consequently, the area at which hydrogen gas is demanded is becoming more wide spread geographically also than before. This tendency would be more increased by spreading fuel cell as a method for power generation which is expected in the future.
One conventional method for obtaining hydrogen gas is to gather hydrogen gas produced as a by-product in chemical plant or to use COG produced at an iron mill as a source of hydrogen. Then hydrogen gas is recovered and pressurized to filling in vessels to be transported. In such cases, cost is not enough low and the amount of hydrogen gas to be supplied does not satisfy the increasing demand. Further, because the chemical plants and iron mills are located near the market place of their main product, the location is generally far from the area where hydrogen gas is demanded and need of transportation pushes up the cost of hydrogen gas.
On the other hand, chemical plants for producing methanol are being constructed in oil-producing or natural gas-producing countries, and supply of methanol is expected to increase in the future. Therefore, a method for producing hydrogen gas using methanol as a raw material is becoming important. Methanol is convenient for using it as a raw material compared to other materials such as naphtha and butane because the reaction temperature of methanol is lower than that of the others. The reaction temperature of methanol is approximately between 250.degree. C. and 300.degree. C. while that of naphtha and butane is higher than 600.degree. C. One of the strong points of this method is that the hydrogen gas is obtained where the gas is demanded by constructing a plant in that location so as to minimize the transportation cost. The amount of hydrogen gas to be obtained may be far larger than the above method in the light of the large amount of menthol to be produced.
A conventional steam reforming method of methanol for obtaining hydrogen is described with reference to FIG. 3. According to the figure, methanol fed through a feed line 1 is pressurized by a pump 2, pure water fed through a feed water line 3 is pressurized by a pump 4, the methanol and the pure water under pressure are mingled together and supplied to a preheating section 5a. The mixture of methanol and pure water, then, passes through a vaporization section 5b, a superheating section 5c, and becomes a superheated stream. The superheated stream is fed to a reformer section 6 and produces hydrogen gas and carbon dioxide as it passes through reformer tubes 7,7 under catalytic effects of a catalyst retained therein. A mixture of hydrogen and carbon dioxide is fed to the pre-heater 5a for heat-exchange with the mixture of methanol and pure water, and led to a cooling section 8 to be cooled by cooling water supplied through a cooling water line 9 for condensation of the excess water contained therein. The mixture is led to a gas-liquid separator 10 for separating the water and gas containing hydrogen. The gas containing hydrogen is led from the separator 10 to an apparatus for the adsorption separation 12 for separating out the hydrogen and the accompanying products. A high purity hydrogen gas and impurities are fed through lines 13 and 14 respectively. The water extracted at the gas-liquid separator 10 is returned to the feed water line 3 for a re-circulation.
On the other hand, a heat transfer oil for regulating the temperature of each section circulates as follows.
First, the oil is heated by a burner 16 as it passes through a heater device 15. Then, the oil is led through a feed oil line 17. The feed oil line 17 is separated in two lines 18 and 19. The line 18 is connected to the super heating section 5c and the vaporizing section 5b therethrough. The line 19 is connected to the reformer 6. The oil is extracted from the vaporizing section 5b and the reformer 7, mingled together, and returned to the heater means 15 through a pump 21.
The reaction whereby the mixture of methanol and water (aqueous solution of methanol) are reformed to produce hydrogen is described, generally, as follows. EQU CH.sub.3 OH+H.sub.2 O.dbd.3H.sub.2 +CO.sub.2 -11.2Kcal/mol
The reaction includes the following two successive reactions, a decomposition process and a conversion process. EQU CH.sub.3 OH.dbd.2H.sub.2 +CO-21.66Kcal/mol (decomposition process) EQU H.sub.2 O+CO.dbd.H.sub.2 +CO.sub.2 +9.84Kcal/mol (conversion process)
As shown by the above chemical equations, the decomposition process tends to proceed effectively in an elevated temperature because the reaction is endothermic, and the conversion process tends to proceed in a lower temperature because the reaction is exothermic. Catalyst for these processes, mainly oxides of chromium, zinc or copper with additives, are proposed by many patents, for example, Japanese Patent Kokoku (second publication) No. 54-11274, Japanese Patent Kokai (first publication) No. 49-47281, 59-131501, 61-234939, 61-234940 and 61-234941). All these prior publications are intended to decrease the reaction temperature of the decomposition process and to increase the reaction rate. But, as far as the above prior art takes it a premise to proceed the decomposition process and the conversion process at a same temperature, it is difficult to improve this two processes at the same time from the view point of chemical equilibrium. However, for obtaining hydrogen in maximum generation, the decomposition of methanol and conversion of carbonoxide are both needed to increase.
One possible solution for improving both processes is to increase the amount of water contained in the mixture as a material, higher than the amount needed in the stoichiometry. But, as the method inevitably increases the cost because of the energy for evaporating the excess water, the ratio of water and methanol in molecular gram has to be between 1 and 1.5 from an economical point of view, which does not improve the reactivity enough drastically. Another possible solution is to maintain the temperature of the decomposition process higher than that of the conversion process by setting up the different temperature at upper zone and lower zone of the catalyst bed. Though the solution may be derived from the above-mentioned nature of the reactions, there has not been provided a pertinent practical method or system which enables the above temperature control. A system conventionally proposed along the line, in a Japanese Patent application Kokai (first publication) No. 49-47281, is explained as follows.
In the system, the reformer section comprises a high temperature chamber and a low temperature chamber which are connected in series by a piping through which the reactants flow. This construction is based on the fact that the reformer generally comprises a tube bundle composed of a large number of reformer tubes and it is difficult to regularly flow water through the space formed between the tubes for cooling the tube uniformly. In the prior art, the piping has a nozzle for receiving supplementary cooling water. Water is added to the reactant flowing through the piping to lower the temperature before the reactants flow in the low temperature chamber. But it is difficult either to regulate the amount of water fed to each of the reformer tubes by this scheme and the scheme, consequently, makes the composition of the reactant unstable. Therefore, in the tubes to which an excessive amount of the cooling water is supplied, temperature of the reactant becomes lower than a optimum level and the activity of the catalyst is hindered thereby. On the other hand, in the tubes to which only insufficient amount of the cooling water is added, the temperature becomes excessively high and the catalyst becomes inactive either. Thus in total, the scheme does not increase the reactivity of the conversion process as expected.